1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a recording material by electrophotography, and more particularly, to an image forming apparatus to which a belt unit having a belt stretched by a plurality of stretching members is removably mounted.
2. Description of the Related Art
In known electrophotographic image forming apparatuses, a toner image on a photosensitive drum is electrostatically transferred onto a belt (or a recording material on the belt) that is stretched by a plurality of tension rollers serving as stretching members. In these image forming apparatuses, it is known that the belt and the tension rollers are combined into one belt unit and the belt unit is removably mounted in the main body of the image forming apparatus.
Japanese Patent Laid-Open No. 2001-139176 discloses a structure for positioning such a belt unit relative to the apparatus body. In this structure disclosed in the publication, positioning is performed by directly pressing two tension rollers, by which a surface of a belt facing a photosensitive drum is stretched, against positioning portions provided in a frame of the apparatus body so that the tension rollers engage with the positioning portions.
In this case, one of the two tension rollers has space with respect to the corresponding positioning portion in the frame of the apparatus body. This prevents interference of the center distance between the two tension rollers with the positioning portions in the frame of the apparatus body. Since the center distance between the tension rollers is determined by the positioning portions in the frame, an error of alignment of the tension rollers can be minimized.
Unfortunately, in the above-described publication, during replacement of the belt unit, the belt is tensioned by the tension rollers. In the belt unit, one bearing portion is fixed to the interior of the belt unit, and another bearing portion is biased by the tension of the belt in a direction to decrease the center distance between the tension rollers. For this reason, when replacing the belt unit, it is necessary to engage the tension rollers with the positioning portions of the frame while increasing the center distance between the tension rollers against the tension. This may reduce operability during replacement of the belt unit. Moreover, when the tension rollers are engaged with the positioning portions in the frame of the apparatus body in a state in which the center distance therebetween is reduced, they may be worn by rubbing against the positioning portions because of the tension.